Traditionally, digitally controlled color printing capability is accomplished by one of two technologies. Ink is fed through channels formed in the printhead. Each channel includes a nozzle from which droplets of ink are selectively extruded and deposited upon a medium. Typically, each technology requires separate ink delivery systems for each ink color used in printing. Ordinarily, the three primary subtractive colors, i.e. cyan, yellow and magenta, are used because these colors can produce, in general, up to several million shades or color combinations.
The first technology, commonly referred to as “droplet on demand” ink jet printing, selectively provides ink droplets for impact upon a recording surface using a pressurization actuator (thermal, piezoelectric, etc.). Selective activation of the actuator causes the formation and ejection of an ink droplet that crosses the space between the printhead and the print media and strikes the print media. The formation of printed images is achieved by controlling the individual formation of ink droplets, as is required to create the desired image. Typically, a slight negative pressure within each channel keeps the ink from inadvertently escaping through the nozzle, and also forms a slightly concave meniscus at the nozzle helping to keep the nozzle clean.
Conventional droplet on demand ink jet printers utilize a heat actuator or a piezoelectric actuator to produce the ink jet droplet at orifices of a print head. With heat actuators, a heater, placed at a convenient location, heats the ink to cause a localized quantity of ink to phase change into a gaseous steam bubble that raises the internal ink pressure sufficiently for an ink droplet to be expelled. With piezoelectric actuators, a mechanical force causes an ink droplet to be expelled.
The second technology, commonly referred to as “continuous stream” or simply “continuous” ink jet printing, uses a pressurized ink source that produces a continuous stream of ink droplets. Traditionally, the ink droplets are selectively electrically charged. Deflection electrodes direct those droplets that have been charged along a flight path different from the flight path of the droplets that have not been charged. Either the deflected or the non-deflected droplets can be used to print on receiver media while the other droplets go to an ink capturing mechanism (catcher, interceptor, gutter, etc.) to be recycled or disposed. U.S. Pat. No. 1,941,001, issued to Hansell, on Dec. 26, 1933, and U.S. Pat. No. 3,373,437 issued to Sweet et al., on Mar. 12, 1968, each disclose an array of continuous ink jet nozzles wherein ink droplets to be printed are selectively charged and deflected towards the recording medium.
In another form of continuous ink jet printing, for example, as described in U.S. Pat. No. 6,491,362 B1, issued to Jeanmaire, on Dec. 10, 2002, commonly assigned, included herein by reference, stimulation devices are associated with various nozzles of the printhead. These stimulation devices perturb the liquid streams emanating from the associated nozzle or nozzles in response to drop formation waveforms supplied to the stimulation devices by control means. The perturbations initiate the separation of a drop from the liquid stream. Different waveforms can be employed to create drops of a plurality of drop volumes. A controlled sequence of waveforms supplied to the stimulation device yields a sequence of drops, whose drop volumes are controlled by the waveforms used. A drop deflection device applies a force to the drops to cause the drop trajectories to separate based on the size of the drops. Some of the drop trajectories are allowed to strike the print media while others are intercepted by a catcher or gutter.
While conventional thermal stimulation devices are effective in initiating the break off of drops from the liquid streams, the stimulation amplitudes can be relatively low. Under certain conditions it is desirable to employ higher stimulation amplitudes. As such, there is an ongoing need for a thermal stimulation actuator capable of providing higher stimulation amplitudes that is suitable for use in a continuous printer system.